(1) Field of the Invention
This invention relates to a two-layer Al bearing material consisting of an Al bearing alloy layer and a backing metal consisting of a steel sheet or the like. More particularly, the invention concerns an Al bearing alloy, which contains Sn and Pb as well as Si, and in which Si is precipitated as Si precipitate particles having spherical or nearly spherical shape in an Al matrix and also Sn-Pb alloy particles are precipitated in the vicinity of the Si precipitate so that it has improved fatigue-resistant property, anti-seizure property and abrasion-resistant property under high speed, high load running conditions of the automobile, and also a bearing material consisting of the Al bearing alloy and backing metal.
(2) Prior Art
Recently, there are trends for smaller size, lighter weight, lower fuel consumption and higher output of automobile engines. Therefore, the bearing should bear increased load, and also the operating temperature of lubricant is increased. The operating conditions of the bearing thus are becoming increasingly stringent.
Generally, the bearing has a three-layer structure consisting of a backing metal formed from a steel plate or the like, an Al bearing alloy layer formed on the backing metal and a bearing layer formed by means of overlay plating on the Al bearing alloy layer. The bearing of this three-layer structure consisting of the backing metal, bearing alloy layer and overlay plating bearing layer, however, is subject to fatigue or seizure due to temperature elevation of the bearing surface, so that it can not meet the stringent operating conditions noted above. Accordingly, there has recently been a demand for a bearing having a two-layer structure, which does not have any surface bearing layer of overlay plating but supports an engine shaft with a bearing alloy layer formed on a backing metal. At present, however, a bearing having stable performance can not be obtained even by adopting the two-layer bearing structure because Al bearing alloys that have been developed so far can insufficiently meet the stringent operating conditions noted above.
More specifically, in the three-layer bearing having a surface bearing layer formed by overlay plating the intermediate bearing alloy layer consists of such an Al bearing alloy as JIS (Japanese Industrial Standards) H 5402, AJ-1 (10% Sn, 0.75% Cu, 0.5% Ni, Al as balance), or JIS H 5402, AJ-2 (6% Sn, 2.5% Cu, 1.0% Ni, Al as balance) or such Al bearing alloy as SAE 780 (6% Sn, 2% Si, 1% Cu, 0.5% Ni, 0.1% Ti, Al as balance), and its Sn content is comparatively small, typically 10 to 6%. For this reason, a Pb-Sn alloy layer is formed as surface bearing layer by overlay plating.
However, under recent high load, high temperature operating conditions, the surface bearing layer is worn out to result in seizure so that it can no longer withstand use in a comparatively short period of time. With the two-layer bearing having overlay plating layer as surface bearing layer, on the other hand, the bearing alloy layer is, for instance, SAE 783 (20% Sn, 0.5% Si, 1.0% Cu, 0.1% Ti, Al as balance) or like Al alloys with as high Sn content as 20%. Since the Sn content is high, however, the hardness is reduced, and the Al matrix is fragile, so that the bearing can not withstand high load.
In "Jiku-uke Gokin" (Bearing Alloys) by Koichi Mizuno, published by Nikkan Kogyo Shinbun Sha, issued in 1954, pp. 139 it is disclosed to form a bearing alloy layer from a bearing alloy, which has enhanced anti-seizure property provided by increasing the lubrication property with addition of Pb together with Sn. This bearing alloy contains 10% of Sn, 1.5% of Cu and 0.5% of Si and also 3% of Pb. Therefore, it can not withstand the high load as noted above although the lubrication property can be improved to some extent.
An Al alloy which contains Sb incorporated to improve the dispersion of Pb which hardly forms a solid solution with Al, is disclosed in Japanese Patent Publication No. 121, 131/1977. Further, an Al alloys containing Cr added for Al matrix reinforcement and also for preventing coarse Sn particles are disclosed in Japanese Patent Publication No. 18,985/1983. These Al alloys, however, have been developed for the purpose of improving the lubrication property under ordinary running conditions, and they do not exhibit sufficient anti-fatigue property under high load running conditions, under which the lubrication mechanism is quite different from that under the ordinary running conditions.
To meet the lubrication mechanism under high load running conditions, there has been proposed a bearing alloy, which is obtained by adding a large quantity of, e.g., 11% or above, of Si to a Sn-containing Al alloy so that it has dispersed Si precipitates which are coarse in grain and have power.
This bearing alloy is imparted with forging property and creep property by the addition of a large quantity of Si. In addition, the cutting force provided by hard Si precipitate particles has an effect of removing irregularities of the counterpart rotary shaft surface to provide a smooth surface, thus improving the performance of the bearing. More specifically, the surface of a rotary shaft made of spherical graphite cast iron or the like has depressions, which result from detachment of graphite particles at the time of polishing, and raised portions such as burrs or edges formed around the recesses. Under high load running conditions, the surface of the bearing alloy layer is ground by such recesses and raised positions of the rotary shaft, thus resulting in abnormal wear of the bearing. With the bearing alloy having dispersed Si precipitate particles the raised portions of the shaft surface are cut away by the hard Si precipitate particles, thus preventing abnormal wear or seizure.
In order to cut the counterpart shaft surface with the Si precipitates to improve the anti-seizure property, it is necessary to add as large amount of Si as 11% or above. The addition of such large amount of Si, however, rather leads to damage to the shaft surface because of precipitation of coarse Si particles. It also leads to cause seizure and deterioration of the cutting or like machining property because of lack of ductilities of the alloy, which is a serious problem. To cope with these problem there has been proposed a bearing alloy, in which the Si content is reduced to be less than 11%, particularly to 0.2 to 5%, as disclosed in Japanese Patent Publication No. 6.955/1983. With this bearing alloy, however, it is impossible to attain improvement of the anti-seizure property due to cutting force of Si precipitates as noted above under high load running conditions, and an improvement in this respect has been called for.